Better help message for `synth just-build`

[hartzell]

The help message for just-build is confusing.

synth build [ports]       Incrementally build ports based on given list, but
                          asks before updating repository and system
synth just-build [ports]  Like 'build', but skips post-build questions
synth install [ports]     Like 'build', but upgrades system without asking

• build builds things, asks whether to update the repository, maybe updates the repository, asks whether to update the system, and finally maybe updates the system.
• install builds things, updates the repository, and finally updates the system, without asking any questions.

It's not clear to me what the difference is between "skipping post-build questions" and "without asking".

This repository's README and a bit of experimentation convinced me that just-build builds the requested package (and stashes it in .../All but does not update the repository or install the package).

If that is correct, then a more "beginner friendly" text would be

synth just-build [ports] Builds ports, without updating repository or system.

Thanks for such a useful tool!